Commit message: random32: update the net random state on interrupt and activity

This modifies the first 32 bits out of the 128 bits of a random CPU's
net_rand_state on interrupt or CPU activity to complicate remote
observations that could lead to guessing the network RNG's internal
state.

Note that depending on some network devices' interrupt rate moderation
or binding, this re-seeding might happen on every packet or even almost
never.

In addition, with NOHZ some CPUs might not even get timer interrupts,
leaving their local state rarely updated, while they are running
networked processes making use of the random state.  For this reason, we
also perform this update in update_process_times() in order to at least
update the state when there is user or system activity, since it's the
only case we care about.

Reported-by: Amit Klein <aksecurity@gmail.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Eric Dumazet <edumazet@google.com>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
CVE: CVE-2020-16166
CWE: CWE-200
Vulnerability description: The Linux kernel through 5.7.11 allows remote attackers to make observations that help to obtain sensitive information about the internal state of the network RNG, aka CID-f227e3ec3b5c. This is related to drivers/char/random.c and kernel/time/timer.c.

Project: linux
File: random.c

void add_interrupt_randomness(int irq, int irq_flags)
{
	struct entropy_store	*r;
	struct fast_pool	*fast_pool = this_cpu_ptr(&irq_randomness);
	struct pt_regs		*regs = get_irq_regs();
	unsigned long		now = jiffies;
	cycles_t		cycles = random_get_entropy();
	__u32			c_high, j_high;
	__u64			ip;
	unsigned long		seed;
	int			credit = 0;

	if (cycles == 0)
		cycles = get_reg(fast_pool, regs);
	c_high = (sizeof(cycles) > 4) ? cycles >> 32 : 0;
	j_high = (sizeof(now) > 4) ? now >> 32 : 0;
	fast_pool->pool[0] ^= cycles ^ j_high ^ irq;
	fast_pool->pool[1] ^= now ^ c_high;
	ip = regs ? instruction_pointer(regs) : _RET_IP_;
	fast_pool->pool[2] ^= ip;
	fast_pool->pool[3] ^= (sizeof(ip) > 4) ? ip >> 32 :
		get_reg(fast_pool, regs);

	fast_mix(fast_pool);
	add_interrupt_bench(cycles);

	if (unlikely(crng_init == 0)) {
		if ((fast_pool->count >= 64) &&
		    crng_fast_load((char *) fast_pool->pool,
				   sizeof(fast_pool->pool))) {
			fast_pool->count = 0;
			fast_pool->last = now;
		}
		return;
	}

	if ((fast_pool->count < 64) &&
	    !time_after(now, fast_pool->last + HZ))
		return;

	r = &input_pool;
	if (!spin_trylock(&r->lock))
		return;

	fast_pool->last = now;
	__mix_pool_bytes(r, &fast_pool->pool, sizeof(fast_pool->pool));

	/*
	 * If we have architectural seed generator, produce a seed and
	 * add it to the pool.  For the sake of paranoia don't let the
	 * architectural seed generator dominate the input from the
	 * interrupt noise.
	 */
	if (arch_get_random_seed_long(&seed)) {
		__mix_pool_bytes(r, &seed, sizeof(seed));
		credit = 1;
	}
	spin_unlock(&r->lock);

	fast_pool->count = 0;

	/* award one bit for the contents of the fast pool */
	credit_entropy_bits(r, credit + 1);
}

Project: linux
File: timer.c

void update_process_times(int user_tick)
{
	struct task_struct *p = current;

	/* Note: this timer irq context must be accounted for as well. */
	account_process_tick(p, user_tick);
	run_local_timers();
	rcu_sched_clock_irq(user_tick);
#ifdef CONFIG_IRQ_WORK
	if (in_irq())
		irq_work_tick();
#endif
	scheduler_tick();
	if (IS_ENABLED(CONFIG_POSIX_TIMERS))
		run_posix_cpu_timers();
}